It sounds like it may be a speaker issue on your phone. Are you able to hear them if you were to turn the speakerphone on? The first thing that I would do is to turn the phone off, remove the battery, keep out for about 45 to 60 seconds. Put the battery back in and turn the device on. Try using the phone to see if still having the problem. If so the next step will be to reset your device by doing a ##786#(RTN) and entering a msl code to reset. You will need to call sprint technical support to get the msl code and directions to do a RTN on your device. After the RTN, if the phone is still not working then I would have to say it will be a device issue. You may want to take to your closest service and repair center store for a diagnostic test. If your phone is still under manufacture warranty or have TEP(Total Equipment Protection) for your device and the phone is tested and is determined it is a manufacture defect then the store may be able to repair or replace at no charge.
Check out a fun infographic that teaches you how to increase storage, enhance security, improve battery life and clean your device.